Soil erosion from wind and water under cropland can be three to four times higher than under grass or trees on similar soils (Lal and Bruce, 1999; see Fact Sheet 4.4). Thus, erosion control is an important strategy to enhance productivity and sequester SOC. Soil loss prevented in the Loess Plateau region of China by soil conservation measures was estimated at 10.6 billion t for 1995 alone (Nie, 1996). Although estimating the total amount of soil erosion from the world's croplands is difficult, it would be much more difficult to estimate the carbon loss from erosion. Lal and Bruce (1999) estimate that the total amount of SOC displaced by soil erosion annually to be in the range of 0.5 Gt, of which 20 percent may be emitted into the atmosphere (the remainder is re-located on the landscape). A recent detailed study, however, indicates that soil erosion losses have been overestimated (Trimble, 1999). This finding suggests that revised estimates of the role of soil erosion control practices beyond conservation tillage are needed.
In addition to conservation tillage, another strategy to reduce erosion is to plant perennial grasses and legumes, either as a regular phase in an arable farming system or in permanent "set-aside" lands. Not only do these practices reduce erosion, they also favor carbon storage because of reduced soil disturbance and greater allocation of carbon below ground (Table 4-5) (Cole et al., 1997; Feller and Beare 1997; Grace et al., 1997; Neill et al., 1997; Paustian et al., 1997b; Smith et al., 1997b, 1998; Carter et al., 1998; Huggins et al., 1998b).
Improved productivity and conservation tillage typically allow increases in soil carbon at a rate of about 0.3 t C ha-1 yr-1. If these practices were adopted on 60 percent of the available arable land worldwide, they might result in a capture of about 0.27 Gt C yr-1 over the next few decades (Lal, 1997). It is unclear if this rate is sustainable because research shows a relatively rapid increase in carbon sequestration for a period of about 25 years, with a gradual leveling off in about 50 years (Lal et al., 1998). Important secondary benefits of conservation tillage adoption include soil erosion reduction, improvements in water quality, increased fuel efficiency, and increases in crop productivity.
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